This invention relates to a monolithic laser diode (LD) and a photo diode (PD) and more particularly to a method for fabricating a photo diode whose light detection layer is thicker than that of the active layer of the LD formed on a common substrate.
A LD is susceptible to variations of its light output, and therefore a PD is usually packaged in alignment with a rear facet of the LD to monitor the light output of the LD to control the light output of the LD, as shown in FIG. 1. Conventionally, the PD is integrated with the LD in a hybrid structure, and the photo current of the PD is fed to an Automatic Power Control (APC) circuit to stabilize the light output of the LD. However, the alignment of the PD with the LD and the electrical and optical matching between the PD and the LD cause many difficult problems in practical packaging.
Although a monolithic integration of a LD and a PD on a common substrate has heretofore been implemented, as shown in FIG. 2, for solving some of the aforementioned problems, one problem of the prior art devices is that the thickness of the light detection layer of the PD is limited to that of the active layer of the LD which is usually thin to optimize the characteristics of the LD. This is because both layers are simultaneously formed during fabrication of the integrated device. Therefore, the PD has a poor light coupling efficiency due to the thinness of its light receiving layers. It is a principal object of this invention to provide a monolithic LD and PD in which the thickness of the light detecting layer of the PD is greater than the thickness of the active layer of the LD, thereby increasing the area of the light receiving aperture of the PD. Another object is to provide a simple method for fabricating a monolithic LD and PD which uses an epitaxial layer growth rate difference depending on the width of a mesa etched pattern on the substrate.